GDP in the context of a heavily industrialized socialist economy vs a dying imperialist country where finance is dominant is entirely different. Production continued to meaningfully grow in the USSR, while what we see in the US Empire is a decay in the superprofits stolen from the global south and a hollowed out industrial base, AI bubble, etc, none of which applied to the USSR. Additionally, the economy exploded after the dissolution of the USSR, not before.

The USSR was stable. It was growing slower than before for a number of reasons, but it remains true that it didn't collapse, it was killed. Do Publicly Owned, Planned Economies Work? by Stephen Gowens is a good essay on some of the factors at play during the dissolution of the USSR. Socialism works.